1. Field of the Invention
This invention relates to a method and apparatus for measuring the position of a stage which is moved two-dimensionally on a plane and for placing an object on the stage while two-dimensionally positioning the object and, more particularly, to a technique for measurement and positioning which need to be performed with high accuracy for particular operations such as working and inspection in semiconductor device manufacture.
2. Related Background Art
A kind of XY stage for retaining an object and precisely moving the same in each of the directions of two axes (X- and Y-axes) perpendicular to each other is used in various apparatus used for transfer of VLSI patterns, including exposure apparatus (steppers and the like), a apparatus for drawing transfer masks, apparatus for measuring coordinates of mask pattern positions and other apparatus for positioning. For measurement of coordinates of the XY stage, a light wave interferometer (laser interferometer) is employed whose light source is, for example, a He-Ne frequency-stabilized laser capable of continuously oscillating at a wavelength of 633 nm. Laser interferometers put on the market by Hewlett Packard Co., Excel Corp. and Zaigo Inc. are known. Essentially, one laser interferometer enables one-dimensional measurement alone, two identical laser interferometers are therefore prepared for two-dimensional measurement. Two plane mirrors having reflecting surfaces perpendicular to each other are fixed on the XY stage, two beams are respectively projected from the laser interferometers on the two plane mirrors, and the change in the stage position in the direction perpendicular to each reflecting mirror surface is measured to calculate the two-dimensional coordinate position of the stage. The reflecting surfaces of the two plane mirrors respectively extend in the directions x and y according to the stroke of the necessary movement of the stage. These plane mirrors are used as a basis for coordinate measurement, and the reflecting surfaces must have a very high degree of flatness. The measurement resolution of the laser interferometer is about 0.01 .mu.m, and the necessary length of the reflecting surface of each plane mirror is about 250 mm in the case of a stage on which a 6 inch semiconductor wafer is placed. That is, if the whole reflecting surface having a length of 250 mm is inclined or locally curved or non-flat and if the deviation from the reference plane is larger than 0.01 .mu.m, this deviation is taken in the value representing the result of measurement using the laser interferometer. Accordingly, if the curving deviation of the plane mirror is 0.05 .mu.m, the measurement or positioning of the stage position is effected with a curvilinear (or oblique) coordinate system deviated from the ideal orthogonal coordinate system by 0.05 .mu.m. Conventionally, for this reason, the plane mirror is manufactured so as to maximize the flatness, but a degree of unevenness of about 0.02 .mu.m is left owing to manufacture errors. The accuracy enabling limitation of the degree of unevenness to 0.02 .mu.m with respect to the whole of the 250 mm reflecting surface is such that a string stretched between two points spaced part from each other by 100 km sags by only 0.8 cm at the center.
It is of course possible to further improve the accuracy according to the method of working the plane mirror. However, for improvement in the flatness, the manufacture cost is considerably increased. In fact, it is impossible to maintain a flatness of 0.02 .mu.m or less due to deformation of the mirror at the time of fixation on the two-dimensionally movable stage or due to secular changes.
To cope with problem, a curve (unevenness) of a plane mirror fixed on the movable stage may be measured by using a reference plane mirror as a standard. In this case, a reference plane mirror having a shape generally equal to that of the mirror to be measured is placed on the stage generally in parallel with the measured plane mirror, beams are projected from an interferometer on the measured plane mirror and the reference plane mirror perpendicularly to the same, and the curving deviation of the measured plane mirror relative to the reference plane mirror is obtained from the change in the distance therebetween determined by the interference between the reflection beams.
However, the method of using such a reference mirror entails drawbacks described below. The manufacture of the reference mirror requires certain labor and cost. Even if an accurate reference mirror can be manufactured, it is difficult to temporarily mount the reference mirror on the stage, and the mounting operation requires much time. In particular, when the reference plane mirror is attached to the stage, it is necessary to avoid application of excessive stresses to the optical block of the reference plane mirror, which stress may deform the block.
Even if such problems can be solved, complicated calculations are required for measurement of the curving deviations, which makes the measurement operation laborious.
Recently, the level of the positioning accuracy of exposure apparatus incorporating this kind of stage is being increased with the reduction in the resolution line width on submicron order (0.8 to 0.4 .mu.m). Accordingly, the influence of curves of plane mirror therefore becomes considerable.